In the prior art, continuously variable gain devices used in oscilloscopes employ variable gain amplifiers which include generally unbalanced-type and balanced-type variable gain amplifiers, the latter type using a differential amplifier structure. Variable gain amplifiers of the unbalanced type are illustrated in FIGS. 5 and 6 in which similar elements within the circuits shown in the figures are designated by the similar symbol. The circuit shown in FIG. 6 is disclosed in National Technical Report, Vol. 27, No. 1, February 1981. Both of the circuits are are comprised of a differential cascode amplifier. The circuit of FIG. 5 includes, as a gain adjusting means, a variable resistor VR1 which is provided between the collectors of common-emitter circuit transistors Q1 and Q2 whereby a change in a resistance between the collectors can be used to adjust the gain of the circuit. With this circuit, the gain adjustment cannot be provided by way of electric signals. Also, the upper limit frequency of the bandwidth of the circuit is limited by the existence of a large amount of stray capacitance associated with the variable resistor which is a large component.
On the other hand, the circuit shown in FIG. 6 uses, in place of a variable resistor such as VR1, a pair of diodes D1 and D2 facing each other (the diodes give rise to less stray capacitance) combined with a voltage source for drawing a current I1 from the diodes in order to enable the gain of the circuit to be controlled by an electric signal and also to increase the upper limit frequency of the bandwidth. In this case, the diodes D1 and D2 each operate as a variable resistance the value of which depends on the magnitude of the current I1. Also, the circuit of FIG. 6 is provided as disclosed in the above-mentioned document, with resistors R8 and R9 connected in series for the detection of the DC level of a voltage in the amplifier and a DC correction circuit which functions to correct any shift of the DC level caused by the current I1 drawn from the cascode circuit; correction being carried out by means of a correction current fed into the collectors of common-base circuit transistors Q3 and Q4.
Either of the variable gain amplifiers shown in FIGS. 5 and 6 has a variable range of gain and a bandwidth which are correlated to each other. As a result, if the variable gain range is widened by increasing the resistances of the resistors R4 and R5, which are the parameters for determining the width of the range, the AC signal component generated at each of the collectors of Q1 and Q2 would increase with the result that the bandwidth is narrowed due to the Miller effect. On the contrary, if the resistances of the resistors R4 and R5 are made smaller so as to widen the bandwidth, the variable range of gain would be narrowed.
Accordingly, an object of the present invention is to provide a variable gain amplifier which allows the above-described problem to be alleviated by having a decreased degree of correlation between a variable range of gain and a bandwidth.
Further, another object of the present invention is to provide a variable gain amplifier which has improved frequency characteristics while keeping a variable range of gain having a required width.